Phobos, the tiny innermost moon of Mars, is to come under unprecedented scrutiny after Europe's mission to Mars goes into orbit around the Red Planet late in 2003. Mars Express is due to pass within 3000 km of the 22 km diameter moon a few hundred times during its two-year nominal mission lifetime.

"There will be many more opportunities for close fly-bys than during NASA's Viking mission," says Tom Duxbury from NASA's Jet Propulsion Laboratory, a veteran of previous Phobos observations who is helping to plan the Mars Express observation campaign.

At a distance of 3000 km, the High Resolution Stereo Camera (HRSC) on board Mars Express will be capable of taking images with a resolution at least as good as any taken by Viking. At 1000 km, the Infrared and Visible Mapping Spectrometer (OMEGA) will also be switched on to map the mineral composition of the tiny moon's surface; and when the distance is only a few hundred kilometres, the Planetary Fourier Spectrometer (PFS) and the Sub-surface Sounding Radar/Altimeter (MARSIS) will record measurements. The Energetic Neutral Atoms Analyser (ASPERA) will monitor the plasma environment around the orbit of Phobos during all fly-bys.

"Mars Express will provide the first global map of Phobos. This is important because we know one side of this moon well, but the other side is less well-known," says Agustin Chicarro, ESA's Mars Express project scientist. "We should get a complete view of the satellite in terms of its topography, sub-surface and composition. This should help us determine once and for all whether it is a captured asteroid or not," he adds.

Mars Express could generate the most comprehensive suite of observations ever recorded for Phobos. They will help to resolve some of the outstanding puzzles about the Martian moon, many of which came to light during previous observation missions. In addition to Viking, these included the ill-fated Russian mission, Phobos-2, and NASA's Mars Global Surveyor, which is still on mission around the Red Planet. Phobos-2 was due to send a lander to the tiny moon in 1989, but was lost just 100 km above the surface.

Phobos is a world of mysterious origin and destiny. It is light, with a density less than twice that of water, and orbits just 5980 km above the Martian surface. One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids. Data returned by the infrared mapping spectrometer experiment (ISM) on board the Phobos-2 mission supported this view.

"In general the composition matched that of the inner asteroids. But we don't know whether Phobos is primordial or whether it has undergone transformation," says Jean-Pierre Bibring, principal investigator for ISM and also for OMEGA on Mars Express. "We found some tantalising evidence for transformation in the form of interesting minerals at the bottom of a crater, which were absent elsewhere on Phobos, suggesting that the bulk composition is inhomogeneous. I'm very excited about observing Phobos again because OMEGA will have much higher resolution and will be able to determine the surface composition unambiguously and pick out minerals at the bottom of craters, thus determining the bulk composition."

However, there are other ideas about the origin of these two moons. One, favoured by Duxbury, is that they are lightly accumulated ejecta from asteroid impacts on the Martian surface, with Phobos composed of ejecta orbiting Mars faster than the planet rotates and Deimos, whose orbit is further out and orbital motion slower, composed of ejecta orbiting more slowly than the planet rotates.

Another mystery about Phobos concerns the origin of deep grooves on its surface. Some of these radiate from Stickney, the large impact crater that distorts the shape of the moon, and are thought to indicate that the force of the impact nearly shattered the small body. However, others on the central portion of Phobos run almost parallel and have prompted another thought.

John Murray, from the Open University in the United Kingdom, and a co-investigator on the HRSC, thinks that the grooves are strings of impact craters made by ejecta thrown up by major impacts on Mars. He has calculated the velocity of such ejecta and the position and direction of the 'grooves' they might be expected to leave on Phobos. "The direction of the grooves corresponds to what the hypothesis predicts," he says. "There are no grooves on the trailing edge of Phobos, which is also what you'd expect."

However, the leading edge of Phobos has never been imaged in close up before and the appearance of grooves here will be critical to the theory. The HRSC on Mars Express should provide the required images for the first time. "If I'm right, we should see wider chains of grooves crossing at all angles here," says Murray.

[Image 1: http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27332]
The giant Stickney crater on Phobos is clearly visible on this Viking image. One of the most striking features on the 27 km diameter, irregularly shaped Phobos is the presence of grooves over most of its surface. The grooves seem to radiate in all directions from the giant Stickney crater (left) and converge on the opposite side of the satellite at a region close to the Stickney antipode. The grooves are best developed near Stickney, where some measure 700 m across and 90 m deep. However, most of the grooves have widths and depths in the 100 - 200 m and 10 - 20 m ranges, respectively.

[Image 2: http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27333]
Deimos, the companion moon of Phobos, was also imaged by the Viking Orbiter. This computer mosaic of Deimos was made with images acquired from the Viking Orbiter during one of its close approaches to the moon. The 15 km diameter Deimos circles Mars every 30 hours. Scientists speculate that Deimos and its companion moon Phobos were once passing asteroids that were pulled in by the gravity of Mars.

[Image 3: http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27329]
One of the highest resolution images to date of Phobos obtained with the Mars Orbiter Camera. This image of Phobos was taken with the Mars Orbiter camera on the Mars Global Surveyor (MGS) on August 19, 1998, 10 AM PDT. The MGS spacecraft was approximately 1080 km from Phobos at closest approach. This image, about 8.2 km wide by 12 km tall, shows the full field-of-view of the Mars Orbiter Camera (MOC) as spacecraft motion swept across the satellite. The image as shown here has a scale of 12 m per picture element. Credit: Malin Space Science Systems/NASA